Create panel > Shapes > Extended Splines > Object Type rollout > Channel
Create menu > Shapes > Channel
Use Channel to create a closed “C” shaped spline. You have the option to specify the interior and exterior corners between the vertical web and horizontal legs of the section.
All spline-based shapes share these parameters. For explanations, see Splines and Extended Splines.
The Channel shape uses the standard creation methods of Center or Edge. Most spline-based shapes share the same Creation Method parameters. For explanations, see Splines and Extended Splines.

Length—Controls the height of the vertical web of the channel.
Width—Controls the width of the top and bottom horizontal legs of the channel.
Thickness—Controls the thickness of both legs of the angle.
Sync Corner Fillets—When turned on, Corner Radius 1 controls the radius of both the interior and exterior corners between the vertical web and horizontal legs. It also maintains the thickness of the channel. Default=on.
Corner Radius 1—Controls the exterior radius between the vertical web and horizontal legs of the channel.
Corner Radius 2—Controls the interior radius between the vertical web and horizontal legs of the channel.
Note: Take care when adjusting these settings. There are no constraining relationships between them. Therefore, it's possible to set an inside radius (Corner Radius 2) that is greater than the length of the web or width of the legs.